Frequency modulated radar apparatus



Nov. 23, 1948.

Filed Nov. 1o. 1945 3 shuts-Shadi. 2

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Filed Nov. 10. 1945 s. R. coLE 2,454,633

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cam- HT 7017A@ Y Patented Nov. 23, 1948 UNITED STATES PATENT OFFICE raEoUENcY in'rnn mman I Ben R. Cole, Princeton. N. J., assignor to Radio y G orpcration of America, a corporation of Dela- Application Ncvembei- 1o, 1945, serial No. 621,949

Claims. l i My invention relates to radio control apparatus and particularly to radar apparatus of the frequency modulated type. i The invention will be described specifically as applied to apparatus that is designed to direct airborne devices toward an enemy target and torelease bombs at the proper time for hitting the target.-

In one system to which the present invention is applied, radio signals are radiated alternately in two overlapping directive patterns, and the amplitude of the signals reflected from the target with one radiation pattern is compared with that of the signal reflected from the target with the other radiation pattern. The alternate radiation of the two patterns is accomplished by switching the transmitter first to one antenna and then to a second antenna. If the amplitudes of the two signals are equal, the reflecting target lies on the equi-signal line of intersection o f the alternate radiation lobes. If the amplitudes are unequal, the target lies in the lobe from which the greater signal is reflected. In order to provide response to an inequality in amplitude of the reflected signals, a circuit capable of comparing the amplitudes of the signals received during successive periods of radiation in alternate directive patterns is employed. In copending application Serial No. 534,114, filed by William R. Mercer on May 4, 1944,- now Patent No. 2,433,287, which issued December 23, 1947, and entitled Comparator circuits, a circuit for this purpose is described and claimed.

The above-mentioned antenna switching produces transient signals which, while they do not 'interfere with the comparator circuit operation,

do interfere with 'proper operation of the bomb release circuit with the result that the bomb may not be released at the proper time. A bomb release circuit of the type referred to is described and claimed in copending application Serial No. 605,412, filed by Royden C. Sanders, Jr., and Daniel Blitzon July 16, 1945, and entitled Radio devices.

An object of the present invention is to provide an improved method of and means for reducing or eliminating the effects o f antenna switching transients' or the like in a radio bomb release circuit. I

A further' object of the invention is to provide an improved method of and means for reducing or eliminating transient eects in an automatic bomb release circuit of the frequency modulated radar type that utilizes thelDoppler effect.

A further object of the invention is to provide an improved radio system for directing an air- 2 borne device toward a target and for automatically dropping bombs thereon.

A still further object of the invention is to provide an improved method of and means for reducing or eliminating undesired eiects in a frequency-modulated altimeter or radar system which occur during the interval between. increasing frequency and decreasing frequency of the frequency modulated radio wave.

It might be expected that the objects of the present invention could be accomplished by blocking the radio receiver or an amplifier therein' during the occurrence of the switching transients or other undesired effects. This was found to be ineffective because such blocking introduced other transients. `According to the present invention, the frequency counter circuit or circuits are rendered inactive during the occurrence of the undesired effects. It has been found that any difficulty previously caused by switching transients, for example, may be substantially veliminated in this way.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure l is 'a block and circuit diagram of a system embodying the invention,

Figure 2 is a group of graphs that are referred to in explaining 'the invention,

Figure 3 is a diagram showing the geometry of a bomb release problem,

Figure 4 is a graph showing the relationship of slant range to slant speed for bomb release at a particular altitude. and the linear approximation to said re'ationship,

Figure 5 is a graph illustrating the variations in frequency of the signals transmitted and received in the operation of the bomb release portion of the system-of Figure 1,

Figure 6 is a graph illustrating the variations in frequency of the beat signal produced in the operation of the bomb release portion of the system of Figure 1, and

Figure 'l is a circuit diagram illustrating an- Y other embodiment of the invention.V

In the several figures, similar parts an'l graphs are indicated by similar reference characters.

Before describing a specific method or circuit for practicing the invention, the portions of the radar comtrol system as described in the abovey Vmentioned copending applications will be de scribed,

Comparator circuit Referring to Fig. 1, there is shown a frequency 3 i/Y modulated radio control system comprising 'a radio t fnitter I which'is cyclically frequency modulaf, j'by a frequency modulator 3, such as a variablecapacitor unit, in response to a modulated sighal 5 applied thereto from a. wave' The output of the transmitter I is radiated all ternately from a pair of directive antennas 2l and 23. This is accomplished by means of a switch'comprising switch arms 25 and 21 connected together mechanically for operation by means of a cam 29. The cam 29 is connected to the motor I9. After reflection from a target, the frequency modulated signals are received by a directive antenna 3| and applied to a beat frequency detector 33. The frequency modulated signals are also supplied to the detector 33 over a iine 35 directly from the transmitter I, whereby the frequency of the beat output of the detector 33 is proportional to the distance to the target.'

This distance may be indicated by a meter 31 connected to the output circuit of a frequency counter 39, which is connected to the output of the detector 33 through a limiter 4I For obtaining directional information, output signal from the detector 33 is applied to an audio ampliiler 43 connected to an amplitude comparator circuit comprising a rectifier/45, a storage capacitor 41 and synchronous switch 49, and a relay amplier 5I. The output circuit .of the amplifier 5I includes a pair of relays 53 and 55, each provided with a single-pole double-throw contactA arrangements for connecting a battery 51 to a motor 59.

The rectifier 45 comprises a grid leak type detector circuit including an electron discharge tube 6I provided with a grid capacitor 63, grid leak resistor 65, grid current limiting resistor 51, and a load resistor 69. In the illustrated circuit, the tube 6I is of the pentode type, with screen potential provided by a voltage divider including resistors 1| and 13 connected across the anode supply voltage.

The anode of the tube 6I is connected through a resistor to one terminal ofthe capacitor 41. The other terminal of the capacitor 41 is connected to the moving contact 11 of the switch 49, which includes two fixed contacts 19 and 8|. The contact 11 is operated by means of acam 83 connected to the motor I9, in synchronism with the switches and 21. The contact 8| is connected to groundl The contact 19 is connected to the amplier 5I through a low pass filter circuit comprising capacitors 85 and 81 and a resistor 89. A resistor 9| is connected across the capacitor 85, and together with the resistor 89 constitutes a grid leak for the amplifier 5I.

The relays 53 and 55 are adjusted so that the drop out current of relay 53 exceeds the pick-up current of relay 55 by a small amount. For example, the relay 53 may be adjusted to pick up with a current of 4.6 milliamperes'and drop out at 4 milliamperes, while the relay 55 is adjusted to pick up at 2.6 milliamperes and drop out at 2 milliamperes. Thus when the output currentoi' the amplifier 5I is less than 2 milliamperes. both relay armatures are in their lower positions as indicated in solid lines in the drawing. When the current is between 2.6 milliamperes and 4 milliamperes, the amature of the relay 55 will be in its upper position as indicated in dash line, while the armature of the relay 53 will remain in its lower position. When the current is greater than 4.6 milliamperes. the armatures of both relays will be in their upper (dash line) positions. Thus when the output current of the amplifier 5I has a value within the range of 2.6 to 4 milliamperes, the armature of the motor 59 is disconnected from the battery 51 and short-circuited. When the output current is above this range. the motor 59 is connected to run in one direction, and when the current is below 2 milliamperes, the motor is connected to run in the opposite direction." The motor 59 may be mechanically connected to the rudder or other steering mechanism (not shown) of a mobile craft carrying the equipment.

In the operation of the above described system, the motor I9 drives the cam I1 to modulate the transmitter Ian d the cams 29 and 83, to switch the antennas4 and the comparator circut. The cam I1 includes two lobes, while the cams 29 and 83 each include one lobe. The purpose of this arrangement is to provide a complete frequency modulation cycle during each period of transmission on each of the antennas 2l and 23. The amplitude of the beat output oi the detector 33 depends upon the strength of the reflected signal. Hence, if the reflecting target is on the equisignal line of the directive patterns of the antennas 2l and 23, the beat output will not vary in amplitude with the operation of the switches 25, 21 and 49. However, if the target is to the left of the equi-signal line, astronger signal will be received during the time that the antenna 2| is being used for transmission and weaker during the time that the antenna 23 is used, causing the beat output of the detector 33 to have a substantially square wave amplitude envelope.

:The phase of this envelop with respect to the switching cycle is constant. Similarly, if the target is'to the right of the equi-signal line, a

similar square wave envelope will be produced' but of opposite phase.

The rectifier tube 5I is biased by its own grid current. Thus. when the amplitude of the output of the amplifier 43 is low, the average anode current of the tube 6I is high, and vice versa. Inasmuch as the anode current through the tube 6I is necessarily unidirectional, the output voltage appearing across the load resistor 69 is unidirectional, following the envelope of the output of the amplier 43, but in opposite phase. During the time that the antenna 23 is connected to the transmitter I, the capacitor 41 is connected to ground through switch 49. The capacitor 41 is thus charged to a voltage depending upon the lstrength of the reiiected signal, in the polarity indicated by the signs and The value of the resistor 15 is selected with respect to that of the capacitor 41 to provide charging to a value between the peak value and the average value of the voltage across the load resistor 69. When the antenna 2I is connected to the transmitter I,

the capacitor 41 is connected to the input circuit of the amplifier 5I. The voltage drop at this time across the load resistor 69 depends upon the strength of the signal reflected from the target in response to transmission of the antenna 2I.

The voltage at the upper end of the resistor 89 is positive with respect to ground. Thus the voltage applied to the input circuit of the ampliiler i is the diierence between that at the upper end of the resistor 69 and thatl to which the capacitor 41 is charged. If the reflected signal is larger from the antenna 23 than the antenna 2|, the average voltage applied to the amplifier 5| will be positive, since the capacitor 'V41 will be charged to a lower voltage than the voltage existing at the anode of the tube 6l while the antenna 2| is connected. Similarly, the voltage at the input of the amplier 5I will be negative if the reflected signal from the antenna 23 is larger than that from the antenna 2|. When the reected signals are equal, 'th input voltage to. the ampliiler 5I is zero. The amplier 5| is biased by means of a cathode resistor 33 to provide an anode current of approximately three v. rriilliamperes, when the input signal is zero. Thus when the target is oi to the left 'of the equi-signal line, the anode currentis decreased and the armatures of the relays 53 and 55 fall to their lower positions connecting the motor 59 for rotation in one direction, for example, clockwise. When the target is o5 to the right of the equi-signal line, the motor 59 is similarly energized to rotate counterclockwise.

In the described embodiment of the invention, the transmitting antennas are alternately energized to provide overlapping radiation lobes, and the receiving antenna is directed alongthe equisignal line. Although this arrangement is preferred at present, it will be apparent that a single transmitting antenna maybe used, with switched receiving antennas, or both thetransmitting and receiving antenna systems may be switched to `provide overlapping ldirective patterns.

As a matter of practice, it is not possible to arrange the switches 25 and 2l to change the connections of the transmitter I instantaneously from one of the antennas tothe other. The de,-

signer is confronted with the choice of having both antennas connected to the transmitter during the switching period, or having iboth antennas disconnected from the transmitter during the switching period. The former alternative is to be preferred, since the variation of the impedance presented to the transmitter by this method is only 2:1. `A similar choice must be made with reference to the switch 49. vIn this case, the moving contact 'l1 must be disconnected from both of the stationary contacts during the switching period, rai-.her than connected to both of them, since the capacitor 85 would otherwise be discharged during each operation of the switch, producing serious transient disturbances that would affect the comparator circuit operation. It should be noted that thelengths of the periods during which the switch 49 is closed in each of its two positions will aiect only the sensitivity of the system, since the capacitor 41 will not be fully charged within the time during which the contact 11 is connected to either side. However. since the average charging current must equal the average discharge, the calibration is independent of the relative durations of the connection periods. Thus, no serious requirement is vided with equivalent arrangement may be substituted for the relays 53 and 55.

\ Automatic bomb'release circuit Refer to Figure 3. It is assumed that an aircraft at the point P is flying horizontally at a velocity G toward a point M, ldirectly over a target at the point Q, at an altitude H. A bomb released at the altitude H without any vertical velocity will require a time Tf to fall tothe level of the target. f

where g equals Ithe acceleration caused by gravity. 32.2 feet/sec?, approximately.

In order for the Ibomb to strike at the point Q, it mustbe released T: seconds before the craft reaches the point M. The time T which must elapse 'before the craft reaches the point M is If the -horizontal speed G and the horizontal component G of the target range were known, the calculation of T would be a simple matter. The condition for release is: L

placed upon the switch 49 with regard to matchin tube characteristic, etc. A single relay pro- Radio reection equipment does not measure the horizontal distance D, but the true or slant distance R. Similarly, the'horizontal speed G is not measured directly, but i-ts slant component S is measured.` Accordinglmit is necessary to determine the time T in terms of these quantities. -It is apparent from Figure 3 that at great distances or low altitudes from the target, the target depression angle a will be small, and the slant speed and slant range will be nearly the same as the horizontal speed and horizontal range. As the distance is decreased or the altitude increased, the differences between the slant and horizontal speeds and distances will increase.

Referring to Figure 4, the curve ||ll shows the relation between slant speed S and slant range R which corresponds to one particular value of Ti, which in turn is the time of fall Tf from an altitude H1. Thus if a bomb is released from the altitude H1, when R and S are of such values as to satisfy the relationship represented by the curve |0I, the bomb will strike the target.

For every different altitude, there is a diier ent relationship between R and S which must be satisiied for proper release. The curve |0| is thus but one of a family of similar curves. In the presthe curve illl which produces the-smallest maximum error throughout the range Qf slant speeds from S'to S'. ,l

The equation of the line` |03 is (4) 'Rems-Re Where m is the slope of the line:

Rf la!! and Ro is the range intercept at zero speed, as indicated in Figure 4. As stated above; the relations shown in Figure 4 correspond to one specied altitude, Hx. For any other altitude, both m and Re will have correspondingly different values. By setting in the values of m and Ru corresponding to the particular altitude H at which a craft is flying a. substantially correct lrelease may be obtained by satisfying the rel-ationship of Equation (4).

r Referring to Figure 1, the output circuit of the limiter 6| is connected to a pair of frequency responsive circuits comprising averaging cycle counters |99 and IM, respectively. The counter |39 includes a capacitor |53, connected from the llmiter 6| to the anode of a triode |95 and to the cathode of a diode |91. The cathode of the triode |45 is connected to the control grid of an amplier tube I 99. The entire load resistance |51 associated with the tube |99 is connected in its cathode circuit so that it acts asa so-called cathode follower. The anode of the diode |51 is connected to a tap |53 on the resistor |5.

The counter IM includes a capacitor |55 connected from the limiter 4| to the anode of a triode |51 and to the cathode of a diode |59. The anode of the diode |59 is connected to the control grid of the cathode follower tube |49. The cathode of the triode |51 is connected to the upper end of the load resistor at the cathode of the tube |49. The counters |39 and |9| are provided with a common storage capacitor IGI, connected between the control grid of the tube |49 and ground. They also have a common load resistor |63, connected from the grid of the tube |49 to a point |55 on a voltage resistor |93.

The control grid of the counter triode |49 and the control grid of the counter triode |51 are coupled through leads 2|| and 2|2, respectively, to the output circuits of a pair of amplifier tubes 2 3 and 2 i5, respectively so that square wave voltages are applied thereto. These voltages are 180 degrees out of phase with respect to each other and are in synchronism with both the transmitter frequency modulation and the antenna switching as explained hereinafter. The connections are such that the triode |51 is cut oi and the triode |45 is conductive while the frequency of the transmitter is increasing, land the triode v|45 is cut off and the triode |51 is conductive while the` transmitted frequency is decreasing. As will be discussed under the heading Switching transient elimination, the square wave counter switching voltages are given a time relation and* duration such as to eliminate the effects of the switching transients.

The cathode of the cathode follower tube |49 is connected to the cathode of a relay amplifier tube |69. The anode circuit of the tube |99 includes the actuating coil of a relay |1|. The contacts of the relay |1| are connected to the actuating circuit of a bomb release mechanism (not shown). The control grid of the relay tube |69 is by-passed to the cathode through a capacitor |13, and is connected through a resistor |15 to the adjustable contact of auvoltage divider |89.

The adjustment and operation of the system thus far described is as follows:

Owing to the cyclical operation of the switch 9 by the motor I9, the frequency of the output of the transmitter varies cyclically as shown by the solid line 20|' of Figure 5. Some of this output is transferred directly to the receiver detector 33 through the line 35. The greater portion is radiated by the antennas 2| andV 23,. Some of the radiated energy strikes the selected target (not shown), and is reected to the antenna 3|. The time required for the transmitted energy to travel to the target and back to the antenna 3| is proportional to the slant'range R from the aircraft to the target. The variations in frequency of the received energy are accordingly delayed with respect to those of the transmitted energy.l The variation of frequency of the received signal as a function of time, assuming no relative motion between the aircraft and the target, is shown by the dot line 203. The delay Tr is proportional to the range R. v

Now if the range is decreasing at a rate S, the frequency of the received signal will be increased. owing to Doppler effect. Thus the frequency of the received signal will vary with time as shown by the dash line 295. The increase in frequency F is directly proportional to the speed S.

The direct and reflected signals from the transmitter are mixed and detected in the receiver detector 33. The output of the receiver comprises a beat frequency signal, which has a frequency equal to the difference in the frequencies of the two signals applied to the receiver. The frequency of this beat signal varies with time as shown by the solid line graph 296 of Figure 6. The mean value of the beat frequency, indicated by the dash line 201 in Figure 6, is directly proportional to the range R, and is equal to: T

where fm is the modulating frequency in cycles per second, f5 is the sweep width, or range of variation of the transmitter frequency, in megacycles per second, and R is the slant range in feet. The beat frequency varies cyclically above and below its mean value by the amount Fa. During increase of transmitter frequency the beat frewhere in is the mean transmitted frequency (see Figure?) in cycles per second, S is the slant speed in feet per second, and c is the velocity of' wave propagation (the velocity of light) in feet per second. During decrease of transmitter frequency the beat frequency is:

` As mentioned above, the triode |51 of the counter |4| is cut olf during increase of transmitted frequency, and the triode |45 of the counter |39 is conductive. 'During this period, the counter |39 operates to provide an average current in in the direction of the solid arrow through the load resistor |63. This current is:

where kl is a constant directly proportional to the capacitance of the capacitor |43. The counter where kn is a constant directly proportional to the capacitance of the capacitor |55. The common load capacitor |6| averages the pulsations in -the voltage at the control grid of the cathode follower tube |I9so that the voltage between the cathode follower grid and ground is:

(11) a+ 2 Hbf-eet?!- anere o is the botenuai at the point les and ai. is the resistance of the load resistor |63.

The cathode of the tube |49 is maintained at substantially the same potential as the control anni)- 246 c C Rearranging the terms,

The quantities k1, k2, Rz., fm and fo are all constants, determined in accordance with design conperiods are represented siderations. Therefore Equation 12 can be writ- As long as e3 is sumciently large to bias the relay tube |69 to cuto, no current flows through the relay Hl and it remains open. However,

when e: becomes equal to e'a, the voltage at which the tube |69 starts to conduct, the relay I'H is closed. At this time Thus by setting the values of K2 eo-el-e'a Kife and Kif in accordance with the altitude H, the relay' can 'relay tube |69 to conduct.-

, 10 l be made to operate at the proper release time, within the limits of the linear approximation of the corresponding R-S curve.

It is apparent from Equation 15 that m is in'- versely proportional to the band width fs. Accordingly, the proper value of m for any particular altitude H may be obtained by adjustment of the sweep width control tap |11 on the resistor I3. Ro, -as shown by Equation ,16, is also inversely proportional to the band width, and is' directly proportional to the voltage which must be present across the counter load resistor |63 to cause ,fthe irhis voltagefisi/the difference between the total bias from cathode to grid of the relay tube |69, (eo-ei), and the cutoff bias es. and is a function of the setting of the tap `on the resistor |89 which controls Vthe bias e1 at the grid of the relay tube |69. Thus the tap on resistor |89` is employed to control Ro in accordance with the altitude.

Switching transient elimination.

In Fig. l, the antenna switching transients are made ineffective in the bomb release circuit by properly shaping and timing the counter switching voltages as previously mentioned. The way in which this is done will be understood by referring to the graphs of Fig. 2 where the said counter switching voltages are represented by the graphs 2|6 and 2H. The frequency modulation sweep cycle of the transmitter is represented by the graph 2| 8. The antenna switching periods and the audio frequency amplifier switching by the graphs 2| 9 and 22|, respectively. I A

The graph 222 shows the transients that are produced at the time the transmitter is switched from one antenna to the other. Comparing the time relation of the transients 222 and the counter switching voltages 2|6 and 2I1, it'will be seenl the point of change from increasing frequency to-decreasing frequency, or vice versa, which is referred to as the turn-around point is slightly delayed in time with respect to the back edge of the voltage Wave I5. This delay naturally occurs where the' triangular frequency modulation is obtained by use of a driven diaphragm type modulator as described in copending application Serial 4 No. 512,153, led November 29, 1943, in the name of Irving Wolff, and entitled Vibratory mechanical system, now Patent No. 2,445,014, which issued July 13, 1947.

Reference will now be made to the circuit and method for obtaining the counter switching voltages 2lbl and 2|1. Referring toFig. 1, the square wave I5 (Fig-s. 1 and 2) is supplied from the switch 9 over a conductor 223 to an amplier tube 226. The wave I5 is then passed through an integration circuit 225 whereby a voltage represented by the graph 221 (Fig. 2) appears on the grid of the tube 23|. Voltage waves 228 and 229 of opposite polarities are supplied Vfrom the plate and cathode resistors 232 and v233 of the tube 23| to the grids of the amplier tubes 2| 4 and 2|3, respectively.

The grids of the tubes 2| 4 and 2|3 have positive voltage biases applied to them through the escasas one megohm resistors 236 and 231 and through the one megohm resistors 238 and 239, respectively. Hence, .in the absence of signal the grids of tubes 2M and 2|3 are maintained at substantlally zero bias. When the integrated waves 228 and 229 are applied to these grids, the plate currentl of thetubes 2m and 2|3 does not change until the grids go suillcientlynegetive to equal the applied xed positive voltage at which time the plate ycurrents decrease until the grldsk have been driven to cuto and no further ychange in plate current takes place. This results in the counter switching waves 2|6 and 2li (Fig. 2) at the plates of the tubes 2id and 2l3, respectively. As previously stated, the counter switching waves 2|' and 2|1 are supplied over the leads 2|2 and 2li, respectively, to the counters itl and |39, respectively.

As indicated in Fig. 2, the counters Isl and |39 function only during the occurrence of the positive portions oi the switching voltages. A comparison of the switching voltage graphs 2|@ and 2|1 with the switching transient graph 222 showsthat during the interval that a switching transient occurs, neither switching voltage is suf- I iiciently positive to operate a counter. Therefore, both counters are inactive during this interval with the result that the output of the two counters is not aifected by`y the switching transient.

Fig. '7 shows an embodiment of the invention wherein the counter diodes |41 and |69 of Fig.l are replaced by triodes 24| and v242, respectively, which have negative blanking pulses 2&3 applied to their grids. The blanking pulses render the counters |39 and I 4| inactive for the duration of the pulses, 'the duration of each blanking pulse being made to coincide with the duration of the switching transients.

The counter blanking pulses 243 may be pro` duced in various ways. In the example illustrated,the square wave I'B is applied to an lnductance coil 244 in the grid circuit of a vacuum :tube 248. 'I'he coil 244 because of its distributed capacity resonates at some suitable frequency.'

such as one from kilocycles per second (10 kc.) to kc., forv example, when it is shock excited by either the front edge or the back edge of the square wave l5. 'I'he resulting short trains of damped waves are reversed in polarity by the tube 246 and appear as shown at 241 at the input of a multivibrator 240. The multivil brator 248 may be of the well known unsymmetrical type which is biased so it is not selfoscillatory and which is adjusted to produce a` narrow negative pulse in response to being triggered by a positive half cycle oi a wave train 247|.

It should be understood that the invention is not limited to the feature of blanking a frequency counter in a frequency-modulated radar system for the purpose specifically described in the foregoing pages. For example, in some systems the antenna switching and amplitude comparator feature may be omitted and only the automatic bomb release apparatus employed. In that case there are, of course, no switching transients, but the use of the counter blanking during the turn-around" time may still be desirable because of transients produced in the audio frequency amplifier by the change from frequency modulation in one direction to that in the other direction. Such transients nor-l mally are not as severe as switching transients to the tendency of the counter to miss counts during the turn-around" period because of their low frequency and their resulting attenuation where the audio frequency ampliiier is designed with the usual low frequency cutoff. At the same time, no new difilculty is introduced as would be the case if an attempt were made to blank an amplifier, instead of the counter.

I claini as my invention:

l. In'a, radio control system including radio transmitter means and radio receiver means whereby signals are radiated to and received from a reflecting object, Asaid control system including a pair of directive antennas having angularly displaced directive radiation patterns. means for frequency modulating said transmitter by a triangular wave and switching means for cyclically changing the directive response of said antenna system from one to the other of two alternate overlapping directive patterns whereby switching transients are unavoidably produced, means for comparing the amplitude of the output of said receiver means during periods of operation with one of said directive patterns with the amplitude of the output of said receiver means during periods of operation with the Vother of said directive patterns, means for supplying the output of said amplitude comparison circuit to a utilization circuit. a pair of frequency counters having a, common storage capacitor, one of said counters being a negative counter and the other being a positive counter whereby the voltage across said capacitor 'is a measure of the difference in the outputs of said counters, and means for making one of the counters lactive only during the upsweep of said triangular wave and for making the other counter active only during the downsweep of said triangular wave and for making both of said counters inactive during the presence of said switching transients.

2. In a radio control system, a radio transmitter, means for cyclically frequency modulating said transmitter to produce a signal that cyclically increases and decreases in frequency, means for radiating said modulated signal, means for receiving said signal after reflection from a target or other object, said receiving means including a detector, means for mixing in said detector said reflected signal with signal obtained directly from said transmitter to produce a beat frequency signal, said system including a pair of directive antennas having angularly displaced directive radiation patterns, switching means for alternately connecting said antennas to said system in synchronism with said cyclic frequency modulation, said switching means producing undesirable switching transients, a pair offrequency counters having a common storage capacitor and which are connected to, supply charging currents in lpolarity opposition to said common storage cabut if the audio frequency amplifier has a fairly pacitor, means for supplying the output of said detector to said counters, means for making one of the counters active only during the increase in frequency of said modulated signal and for making the other counter active only during the decrease in frequency of said modulated signal and radiating said modulated signal, means for receiving said signal after reflection from a target or other object, said receiving means including a detector, means for mixing in said detector said reflected signal with signal obtained directly from said transmitter to produce a beat frequency signal, said system including a pair of directive aners active only during the increase in frequency of said modulated signal and for making the other counter active only during the decrease in frequency of said modulated signal and for making both of said counters inactive during the presence of said switching transients, said last means comprising wave shaping means for producing from said square wave a counter switchingwave having positive and negative periods with blanking periods therebetween and means for applying'said switching wave to said counters.

5. In. a radio control system. a radio transmitter, means for cyclically and linearly Vfrequency modulating said transmitter to produce a signal that cyclically increases linearly in frequency, means for radiating said modulated signal, means for receiving said signal after reflection from a tennas having angularly displaced directive ratarget or other object, said receiving means in.- cluding a detector, means for mixing in said detector said reflected signal with signal obtained directly from` said transmitter to produce a beat frequency signal, said system including a pair patterns with the amplitude of the output of said t detector during periods of operation with the other of said directive patterns, means for supplying the output of said amplitude comparison circuit to a utilization circuit, a pair of frequency crease in frequency of Asaid modulated signal and for making the other counter active only during the decrease in frequency of said modulated signal, and means 'for making both of said counters inactive during the presence of said switching transients.

oi' directive antennas having angularly displaced directive radiation patterns, switching means for connecting said antennas alternately to said system in synchronism with said cyclic frequency modulation, said switching means Iproducing undesirable switching transients, means for comparing the amplitude of the output of said detec-'1 tor during periods of operation with one of said directive patterns with the amplitude of the output of said detector, during periods of operation with the other of said directive patterns, means for supplying the output of said amplitude comparison circuit to a-utilization circuit, a pair of frequency counters which have a common storage capacitor and which are connected to supply charging currents in polarity opposition to said common storage capacitor, each counter including a pair of vacuum tubes each having a control grid, means for supplying the output of said detector to said counters. means for making one Y 4. In a radio control system, a radio transmit- I of the counters active only during the increase in frequency of said modulated signal land for making the other counter active only during the decrease in frequency of said modulated signal,

from said square wave, means for cyclically frequency modulating said transmitter by said triangular wave to produce asignal that cyclically increases and decreases in frequency, means for radiating said modulated signal, means for receiving said signal after reflection from an object, said receiving Vmeans including a detector, means for mixing in. said detector said reflected signal with signal obtained directly from said transmitter to produce a beat frequency signal, said system including a pair of directive antennas having angularly displaced directive 4 radiation patterns, switching means for alternately con.- necting said antennas to said system in synchronism with said cyclic frequency modulation, said switching means producing undesirable switching transients, a pair of frequency -counters having a common storage capacitor and which are connected to supply charging currents in polarity opposition to said common storage capacitor, means for supplying the output of said detector to said counters, means for making one of the 'countsaid last-mentioned means including means for supplying a rectangular voltage wave to the grid of a tube in each of said counters', and means for making both of said counters inactive during the presence of said switching transients, said last means including means for producing blanking pulses and means for applying said blanking pulses to the grids of the other vacuum. tubes in said counters.

' BEN R. COLE.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name lliate 2,206,637 f Koch July 2, 1940 2,206,896 Higgins July 9, 1940 2,232,858 Lane Feb. 25, 1941 

